1. Technical Field
This invention relates to the art of making ductile or semiductile cast iron and particularly to a method for enhancing the machinability of such irons while retaining or improving other physical characteristics.
2. Description of the Prior Art
Ductile iron, in the molten form, is that which has been subjected to graphite modifiers to stimulate the formation of spheroidal graphite in the solidified iron, and semiductile iron is that typically referred to as compacted graphite iron and utilizes basically the same chemistry as that for ductile iron, but the graphite modifiers are added in smaller amounts or for different periods of time so as to not fully effect a total conversion to spheroidal graphite. Such ductile or semiductile irons are produced by the use of commercial graphite modifiers in the form of magnesium or cerium, the latter being as additions in very small regulated amounts to the melt prior to solidification. When the magnesium or cerium content in the solidified structure is about 0.025%, nodular or spheroidal graphite usually precipitates. Flake graphite is formed at magnesium concentrations below about 0.015%. Accordingly, with magnesium or cerium concentrations in the range of 0.015-0.025% compacted graphite (otherwise sometimes referred to as vermiculite) will form semiductile iron.
Conventional ductile or semiductile irons, after heat treatment to enhance the overall physical characteristics of the irons, may contain small but detrimental quantities of martensite, or if not martensite, then unreacted retained austenite which during machining converts to martensite. The conversion to martensite during machining is detrimental to tool life and to dimensional control of the part being machined. Because of the presence of such martensite, and therefore the difficulty of machining such conventional heat treated irons, such irons are necessarily subjected to heat treatment after machining, which is cumbersome. Such machining would be carried out on the as-cast metal article; this is highly uneconomical, particularly in an automated casting and heat treatment commercial line where such articles or castings must be removed and carried to a machining station and then, when machined, recarried and reinstalled in the heat treatment automated line and back to the machining line, for finish machining.
Such conventional austempered ductile irons or semiductile irons contain generally 3.5-3.8% by weight carbon (all percentages given hereafter will be considered by weight unless indicated otherwise), 2.0-3.0% silicon, 0.2-0.9% manganese, sulphur no greater than 0.015%, phosphorus no greater than 0.06%, molybdenum in the range of 0-0.5%, nickel in the range of 0-3.0%, copper in the range of 0-3.0% as a direct substitute for nickel that would ordinarily be used. A conventional ductile iron will possess a yield strength of 36-73 ksi, typically 65 ksi, a tensile strength of 58-116 ksi, typically 80 ksi, an elongation of 2-15, and a hardness in the range generally of 140-270 BHN.
The austempering treatment, as is well known, is one in which the solidified cast iron is heated to an austenitizing temperature usually about 1600.degree. F. or in excess thereof, and held at this temperature to obtain austenite in the matrix. This will usually require about two hours, but may be in the range of 0.5-4 hours. The austenitized iron is then quenched at a rate sufficient to drop the temperature to the range of about 450.degree.-800.degree. F. to avoid passing through the pearlite nose of a time, temperature, and transformation plot, and holding at such intermediate temperature until the austenite is converted to a harder microstructure such as bainite or high carbon austenite and ferrite. After such conversion, the article is dropped in temperature to ambient conditions by air cooling.
In instances where the nickel and other strengthening alloying agents are introduced to the melt, the physical characteristics have been elevated to the levels of 85-100 ksi for yield strength, 100-130 for tensile strength, 5-7% elongation, and 240-320 BHN for hardness (see U.S. application Ser. No. 647,333, filed 9/4/84, commonly assigned to the assignee of this invention). Theoretically, austempering heat treatment using the presence of 0.25-0.4 molybdenum and 0.5-3% nickel allows the iron to convert to about 65% ferrite and 35% austenite. Some of the austenite converts to martensite and makes it brittle during machining.
It would be desirable if a method could be devised by which a ductile or semiductile iron could be obtained which has yield strengths in excess of 100,000 psi, a tensile strength in excess of 150,000 psi, elongation about 5%, and hardness levels of 260-300 BHN, such cast iron being machinable after heat treatment thereby eliminating the necessity for removing such castings from an automated casting and heat treat line before the machining operation is to be carried out.